As a light emitting material used in a light emitting layer of a light emitting device, a composition is known obtained by doping a host material with a phosphorescent compound showing light emission from the triplet excited state. Since light emission of the triplet excited state attains 4 times light emission efficiency in principle as compared with light emission from the singlet excited state, a light emitting device using a phosphorescent compound is paid to attention as a display device in display application and a planar light source in illumination application.
The above-described display device and planar light source are desired to realize high luminance at low current density (namely, realizing high current efficiency) and to realize low power consumption by reducing driving voltage (namely, realizing high light emission efficiency).
For realizing high current efficiency, it is important that the lowest triplet excited state (hereinafter, referred to as “T1” in some cases) of a host material used in a composition with a phosphorescent compound is located at high energy level.
For realizing high light emission efficiency, it is important that a host material has a high conductive property.
As a host material for a phosphorescent compound, a polymer compound containing as a repeating unit a 1,4-phenylene group having specific substituents at 2-position and 5-position is known (Patent document 1). As another host material, a polymer compound containing as a repeating unit a fluorene-2,7-diyl group and a 1,4-phenylene group having specific substituents at 2-position and 5-position is known (Patent document 2).